1. Field of the Invention
The present invention relates to manufacture of standard gas mixtures and, more particularly, to a process and an apparatus for producing standard gas mixtures for testing, calibrating, . . . highly sensitive analytical instruments such as an atmospheric-pressure ionization mass spectrometer (APIMS).
2. Description of the Background
A variety of gases are used in manufacturing semi-conductor devices such as LSIs. These gases contain impurities. The impurities have an adverse influence on the characteristics of the LSIs. Hence, it is demanded that the gases be as pure as possible. This demand grows stronger, along with the increase in the integration density of LSIs. To meet this demand, a high-accuracy and reliable analysis of gases is required.
The techniques commonly used for analyzing such gases for determining the impurity contents thereof are: gas chromatography, gas chromatography-mass spectroscopy, and Fourier transformed infrared spectroscopy. The detection limit of these techniques are, however, 1 to 10 ppb at best. In view of this, these analytical techniques cannot be said to determine the impurity content of the gases as sensitively as is required for example in the manufacture of LSIs.
Known as apparatus for producing standard gas mixtures for use in the various analytical instruments is a standard gas generator which dilutes a high-concentration gas mixture containing the impurity to be analyzed with a so-called zero gas in one or more stages, thereby generating a standard gas mixture under a predetermined pressure at a predetermined flow rate. This apparatus is very useful in producing standard gas mixture to be used to obtain calibration curves. However, it cannot be employed in the internal standard method or the standard addition method, wherein a known amount of a high-concentration standard gas is added directly to a sample gas. Nor can the apparatus be used to produce a standard gas mixture which contains multiple component gases.
Therefore, it has been demanded that a process and an apparatus be developed which can produce these various gas mixtures.
More generally, most of the following functions are required in the daily practice of gas analysis:
Introduce the gas to be analyzed in a controlled way into the analyzing device; PA1 Add to the gas to be analyzed, calibrated amounts of gaseous species in order to apply the standard addition and internal standard generation methods; PA1 Introduction a blanc or zero gas into the analyzing device; PA1 Add to the zero gas calibrated amounts of gaseous species in order to generate controlled mixtures facilitating the calibration of the analyzing device; PA1 Dilute in a controlled way the gas to be analyzed by the zero gas before introduction of the mixture into the analyzing device. This last point is particularly relevant for very polluted, or toxic and/or corrosive gases. The process and apparatus according to the present invention makes it possible to complete the above functions in a simple and effective way which is specially important when doing ultratrace level analysis. PA1 Controlling the pressure of a raw gas; PA1 Purifying said raw gas, thereby generating a high-purity diluent gas; PA1 Controlling the flow rate of at least one high concentration standard gas; PA1 Mixing the high-purity diluent gas and said at least one high-concentration standard gas, thereby generating a medium-concentration gas mixture; PA1 Dividing the medium-concentration gas mixture into a first flow and a second flow; PA1 Mixing the first flow with a sample gas or the high purity diluent gas, or a mixture thereof, thereby obtaining a standard low-concentration gas mixture; PA1 Controlling the pressure of the sample gas; PA1 Controlling the pressure of said second flow of said medium concentration gas mixture; PA1 Controlling the pressure of said standard low concentration gas mixture. PA1 a plurality of gas sources including a source of raw gas, a source of sample gas and sources of different high-concentration standard gases; PA1 A manifold for gathering the high concentration standard gases from the sources of high concentration standard gases and delivering a high concentration standard gas mixture; PA1 purifying means for purifying the raw gas supplied from said source of raw gas, thereby generating a high purity diluent gas, PA1 dividing means for dividing said high purity diluent gas in a first and second portion; PA1 mixing means for mixing said first portion of said high purity diluent gas with said high concentration standard gas mixture, PA1 dividing means for dividing the medium concentration gas mixture into a first flow and second flow; PA1 means for mixing the second portion of the high purity diluent gas and the sample gas and generating a diluent gas which is either the high purity diluent gas or the sample gas or a mixture thereof; PA1 means for mixing said first flow of said medium concentration gas mixture and said diluent gas generating a standard gas mixture. PA1 a plurality of gas sources including a source of raw gas and sources of different high-concentration standard gases; PA1 a manifold for gathering the high-concentration standard gases from the sources of high-concentration standard gases and delivering a high-concentration standard gas mixture; PA1 a source of sample gas; PA1 a sample gas passage connected to said source of the sample gas; PA1 means for controlling the pressure of the raw gas; PA1 means for controlling the pressure of the sample gas; PA1 purifying means for purifying the raw gas supplied from said source of raw gas, thereby generating a high purity diluent gas; PA1 means for dividing said high purity diluent gas in a first and second portions, said second portion flowing through first restriction means; PA1 mixing means for mixing the first portion of the high-purity diluent gas with the high-concentration standard gas mixture supplied from said manifold, thereby generating an about homogeneous medium-concentration gas mixture; PA1 means for dividing the medium-concentration gas mixture into a first flow and second flow, said first flow being supplied through second restricting means; PA1 means for controlling the flow rate of the second portion of the high purity diluent gas, said means having a closed position wherein no gas flows, and an open position wherein the flow rate of said diluent gas can be controlled; PA1 means for controlling the flow rate of the sample gas, said means having a closed position wherein no sample gas flows, and an open position wherein the flow rate of the sample gas can be controlled. Third restriction means receiving the second portion of the high purity diluent gas and/or the sample gas and generating a diluent gas; PA1 means for mixing said first flow of said medium-concentration gas mixture and said diluent gas for generating a standard low-concentration gas mixture; PA1 means for controlling the pressure of the second flow of said medium-concentration gas mixture; PA1 means for controlling the pressure of said standard low-concentration gas mixture. PA1 Controlling the pressure of the raw gas is preferably carried out either before the purification step, by means of a pressure regulator or after the purification step, by means of a back pressure regulator. PA1 Controlling the flow rate of a high concentration standard gas may be preferably accurately carried out either by means of a mass flow controller or a neddle valve associated to a pressure regulator. PA1 Controlling the pressure of the low concentration gas mixture, i.e. the mixture adapted to flow in the analyzer, such as APIMS, is carried out preferably: